The use of non-contact eddy current coil technology is known for use in detecting defects in such products. The product under test passes through or adjacent to an electrical test coil which has been excited by an alternating current. This induces a flow of eddy currents around the test material, or, in the case of a sector coil, in the area under the coil. Short, intermittent anomalies or flaws cause a variation in the eddy current pattern which the instrument detects.
The minimum flaw length which can be consistently detected is a function of the size of the coil employed and throughput speed of the mill roll and its rotary speed.
Such detection equipment can be mounted onto roll grinding equipment, before, during and after roll grinding activities to effectively detect and quantify surface defects such as roll cracks, bruising and magnetism
The coil oscillation frequency is, put simply, measured, de-modulated to extract defect information from the carrier wave, ac de-coupled and amplified to produce a signal from which the defect can be recognized.
The very nature of defect detection is to detect changes in sensed signals. Consequently, de-modulation has always been critical to enable the extraction of the defect signal from the carrier. Following demodulation filtering has always been used to remove any background noise and changes in frequency caused solely by slight contours in the mill roll. This de-modulation and filtering process requires prior knowledge of the mill roll's diameter in order to work correctly
The Applicant has recognized that this de-modulation of the frequency causes, in many cases, circumferential defects to go undetected as the frequency remains constant across the defect thereby causing the instrument to record only a minute change in frequency at the start of the defect and a further minute change in frequency at the end of the defect. To even the trained eye, the changes appear to be two small (longitudinal) defects, which are hard to see and may even be missed altogether.
Moreover, as known systems of detection are dependent on the rotational and through-speed of the mill roll, precise location of any defect, once detected, is cumbersome and time consuming.